<?php

/* Computes the lightness of a pixel. Lightness is used in HSL notation, as well
 * as in various operations on images. This function returns a normalized value
 * for lightness between 0 and 1, inclusive (0.0 being black, 1.0 being white).
 *
 * double imagelightnessat(resource img, int x, int y)
 *        img         An open image resource to operate on (true color or palette)
 *        x, y        The coordinates of the pixel to work on
 *
 * by Jed Smith <?php $u = "jed"; $d = "bz"; printf("<%s@%s.%s>", $u, $u, $d) ?>
 */ 
function imagelightnessat($img, $x, $y)
{
    if(!is_resource($img))
    {
        trigger_error("imagelightnessat(): supplied argument is not a valid "
            . "Image resource", E_USER_WARNING);
        return 0.0;
    }
    $c = @imagecolorat($img, $x, $y);
    if($c === false) return false;
    if(imageistruecolor($img))
    {
        $red = ($c >> 16) & 0xFF;
        $green = ($c >> 8) & 0xFF;
        $blue = $c & 0xFF;
    }
    else
    {
        $i = imagecolorsforindex($img, $c);
        $red = $i['red'];
        $green = $i['green'];
        $blue = $i['blue'];
    }    
    $m = min($red, $green, $blue);
    $n = max($red, $green, $blue);
    /* Because RGB isn't normalized in GD, we divide by 510 here.
     *  Lightness = (Max(RGB) + Min(RGB)) / 2
     * But that's assuming red, green, and blue are 0 through 1 inclusive.
     * Red, green, and blue are actually 0-255 (255 + 255 = 510).
     */
    $lightness = (double)(($m + $n) / 510.0);
    return($lightness);
}

?>